In the field of exploration geophysics, seismic data is typically recorded through the use of active seismic sources, such as air guns, vibrator units, or explosives, and receivers, such as hydrophones or geophones. The sources and receivers may be arranged in many configurations. Typically, a seismic survey is designed to optimize the source and receiver configurations so that the recorded seismic data may be processed to analyze and/or locate subsurface geologic features of interest such as hydrocarbon reservoirs.
In a seismic reflection survey, seismic energy travels as an outwardly propagating wavefront through a subsurface geological structure. This energy will reflect from an interface between different rock layers before being recorded as a seismic trace by a receiver. The seismic trace is a graph of the variation of seismic amplitude versus time. The seismic amplitude depends on an angle of incidence, a density variation and a change in primary wave (e.g., P-wave, or compressional wave) and secondary wave (e.g., S-wave, or shear wave) velocity across the interface between different rock layers.
In some cases, it is desirable to analyze the recorded seismic amplitudes. This may be done in many ways. One step in conventional processing of seismic reflection data involves adding multiple seismic traces that share a common mid-point, but have different source-receiver offsets. This is commonly called “stacking”. Stacking generally improves the signal to noise ratio, but can result in ambiguity surrounding the cause of the seismic amplitudes. For example, a high seismic amplitude could indicate either the presence of fluids or the presence of a particular lithology.
One conventional technique that can provide an improved method of delineating between lithology and fluids is employment of amplitude variation with offset (AVO) or angle (AVA) for a representative offset/angle gather. Those of skill in the art would be aware that amplitude variation with angle (AVA) is often used interchangeably with amplitude variation with offset (AVO).
During processing, this type of AVA data may not be stacked thereby to preserve information that can be used to distinguish indicators of fluids from indicators of lithology. For example, considering a seismic trace, in one scenario, a hydrocarbon-bearing sand may generally have an increasingly negative seismic amplitude at further source-receiver offsets compared to a water-bearing sand which may be indicated by a decrease in positive seismic amplitude at further source-receiver offsets.
The above methods may however often be biased and may not truly represent the geologic features. In addition, conventional methods may fail where seismic data quality is low, such as where random and/or coherent noise is prevalent, or where seismic gathers are not flat.
There is a need for seismic processing methods capable of producing improved AVA information that may be used for analysis of geologic features of interest.